Transmission Hydraulic Control System

ABSTRACT

An automatic transmission uses 6 shift elements applied in combinations of four to establish ten forward speed ratios and a reverse speed ratio. The automatic transmission uses a hydraulic control system to control engagement of the six elements, control engagement of a torque converter bypass clutch, control engagement of a parking pawl, and to provide fluid for a hydrodynamic torque converter and for lubrication. The parking pawl is disengaged in response to engagement of two of the six shift elements and remains disengaged in response to engagement of other shift elements. A single valve controls several different functions associated with the two-pass torque converter. Pressurized fluid is provided by a variable displacement engine driven pump and also by an electric pump. A priority valve reduces lubrication flow when other fluid demands are high as indicated by the pump displacement control circuit.

TECHNICAL FIELD

This disclosure relates to the field of hydraulic control systems forautomatic transmissions for motor vehicles.

BACKGROUND

Many vehicles are used over a wide range of vehicle speeds, includingboth forward and reverse movement. Some types of engines, however, arecapable of operating efficiently only within a narrow range of speeds.Consequently, transmissions capable of efficiently transmitting power ata variety of speed ratios are frequently employed. When the vehicle isat low speed, the transmission is usually operated at a high speed ratiosuch that it multiplies the engine torque for improved acceleration. Athigh vehicle speed, operating the transmission at a low speed ratiopermits an engine speed associated with quiet, fuel efficient cruising.Typically, a transmission has a housing mounted to the vehiclestructure, an input shaft driven by an engine crankshaft, and an outputshaft driving the vehicle wheels, often via a differential assemblywhich permits the left and right wheel to rotate at slightly differentspeeds as the vehicle turns.

Discrete ratio transmissions are capable of transmitting power viavarious power flow paths, each associated with a different speed ratio.A particular power flow path is established by engaging particular shiftelements, such as clutches or brakes. Shifting from one gear ratio toanother involves changing which shift elements are engaged. In manytransmissions, the torque capacity of each shift element is controlledby routing fluid to the shift elements at controlled pressure. Acontroller adjusts the pressure by sending electrical signals to a valvebody.

In addition to controlling the torque capacity of the shift elements,the valve body provides fluid for other purposes. These includeproviding fluid for lubrication and providing fluid to a torqueconverter. The fluid absorbs heat that is generated by friction withinthe transmission. To regulate the temperature of the transmission fluid,the fluid is routed through a heat exchanger.

Typically, the fluid is pressurized and circulated by an engine drivenpump. However, some vehicles automatically shut off the engine whenpower is not required in order to reduce fuel consumption. Some of thefunctions provided by the fluid must be maintained during these periodsof time.

When a vehicle is parked, the transmission may engage a parking pawlwhich holds the transmission shaft stationary to prevent the vehiclefrom rolling. The parking system is designed to remain engaged withoutconsuming any power during extended unattended periods. Normally, theparking pawl is engaged in response to the driver selecting Park and isdisengaged in response to the driver selecting any other range, such asReverse, Neutral, Drive, or Low. However, there are some conditions inwhich the transmission may over-ride the driver selection.

SUMMARY OF THE DISCLOSURE

A transmission includes three shift elements engageable to establish areverse ratio and a fourth shift element engageable in combination withthe first and second shift elements to establish forward launch ratio. Apark valve engages a park pawl in response to simultaneous engagement ofthe first and second shift elements and maintains the parking pawl inthe disengaged state as long as the second or third shift elements areengaged. The transmission may also include a fifth shift element notengaged in either the reverse ratio or the forward launch ratio and asixth shift element engaged in both the reverse ratio and the forwardlaunch ratio. The park valve maintains the disengaged state as long asthe fifth shift element or sixth shift element is engaged.

A method of controlling a transmission with six shift elements includesengaging first, second, and third shift elements while Park is selectedand engaging a fourth shift element in response to the shift selectorbeing moved out of the Park position in order to cause disengagement ofthe parking pawl. When the shift selector is moved to the Reverseposition, the controller may first engage a fifth shift element, thenengage the fourth shift element, then release the second shift element.When the shift selector is moved to the Drive position, the controllermay first engage a sixth shift element, then engage the fourth shiftelement, then release the second shift element. In this way, five shiftelements are engaged as the parking pawl is released. When the shiftlever is returned to the Park position, the method may include reducingthe torque capacities of the first, second, and third shift elementscausing the parking pawl to re-engage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a transmission system.

FIG. 2 is a schematic diagram of a transmission gearing arrangementsuitable for the gearbox of the transmission system of FIG. 1.

FIG. 3 is a high level schematic diagram of a hydraulic control systemsuitable for use with the transmission system of FIG. 1.

FIG. 4 is a schematic diagram of a fluid supply subsystem of thehydraulic control system of FIG. 3.

FIG. 5 is a schematic diagram of a first portion of a clutch controlsubsystem of the hydraulic control system of FIG. 3 suitable for use tocontrol four of the shift elements of the gearing arrangement of FIG. 2.

FIG. 6 is a schematic diagram of a second portion of a clutch controlsubsystem of the hydraulic control system of FIG. 3 suitable for use tocontrol two of the shift elements of the gearing arrangement of FIG. 2.

FIG. 7 is a schematic diagram of a park subsystem of the hydrauliccontrol system of FIG. 3.

FIG. 8 is a schematic diagram of a first portion of a converter/lubecontrol subsystem of the hydraulic control system of FIG. 3.

FIGS. 9a, 9b, and 9c illustrate a spool valve, in three positionsrespectively, suitable for use as the torque converter regulator valveof the fluid supply subsystem of FIG. 8.

FIG. 10 is a schematic diagram of a second portion of a converter/lubecontrol subsystem of the hydraulic control system of FIG. 3.

DETAILED DESCRIPTION

Embodiments of the present disclosure are described herein. It is to beunderstood, however, that the disclosed embodiments are merely examplesand other embodiments can take various and alternative forms. Thefigures are not necessarily to scale; some features could be exaggeratedor minimized to show details of particular components. Therefore,specific structural and functional details disclosed herein are not tobe interpreted as limiting, but merely as a representative basis forteaching one skilled in the art to variously employ the presentinvention. As those of ordinary skill in the art will understand,various features illustrated and described with reference to any one ofthe figures can be combined with features illustrated in one or moreother figures to produce embodiments that are not explicitly illustratedor described. The combinations of features illustrated providerepresentative embodiments for typical applications. Variouscombinations and modifications of the features consistent with theteachings of this disclosure, however, could be desired for particularapplications or implementations.

FIG. 1 schematically illustrates a vehicle transmission. Bold solidlines represent mechanical power flow connections. Thin solid linesrepresent the flow of hydraulic fluid. Dashed lined represent the flowof information signals. Power is supplied at input shaft 10, generallyfrom an internal combustion engine crankshaft. Fluid coupling 12includes an impeller driveably connected to input shaft 10 and a turbinedriveably connected to turbine shaft 14. Power is transmitted from theimpeller to the turbine via moving fluid whenever the impeller rotatesfaster than the turbine. Fluid coupling 12 may be a torque converterwhich also includes a stator which redirects the fluid when the impelleris rotating substantially faster than the impeller such that the turbinetorque is a multiple of the impeller torque. Gearbox 16 includes gearingand shift elements configured to establish various power flow pathsbetween turbine shaft 14 and output shaft 18. Each power flow path maybe established by engaging an associated subset of the shift elements.At low vehicle speed, a power flow path providing torque multiplicationand speed reduction between the turbine shaft and the output shaft maybe established to optimize vehicle performance. At higher vehiclespeeds, a power flow path providing speed multiplication may beestablished to minimize fuel consumption.

The shift elements within gearbox 16 are engaged by supplying hydraulicfluid at an elevated pressure to a clutch apply chamber. Each shiftelement may include a clutch pack having friction plates splined to onecomponent interleaved with separator plates splined to a differentcomponent. The fluid forces a piston to squeeze the clutch pack suchthat frictional force between the friction plates and the separatorplates couples the components. The torque capacity of each shift elementvaries in proportion to changes in the fluid pressure. Pump 20, drivenby input shaft 10, draws fluid from sump 22 and delivers it at anelevated pressure to valve body 24. Valve body 24 delivers the fluid tothe clutch apply chambers at a pressure controlled in accordance withsignals from powertrain controller 26. In addition to the fluid providedto clutch apply chambers, valve body provides fluid for lubrication andprovides fluid to torque converter 12. The fluid eventually drains fromgearbox 18 back to sump 22 at ambient pressure.

An example transmission is schematically illustrated in FIG. 2. Thetransmission utilizes four simple planetary gear sets 30, 40, 50, and60. Sun gear 36 is fixedly coupled to sun gear 46, carrier 32 is fixedlycouple to ring gear 68, ring gear 48 is fixedly coupled to sun gear 56,ring gear 58 is fixedly coupled to sun gear 66, turbine shaft 14 isfixedly coupled to carrier 42, and output shaft 18 is fixedly coupled tocarrier 62. Ring gear 38 is selectively held against rotation by brake70 and sun gears 36 and 46 are selectively held against rotation bybrake 72. Turbine shaft 14 is selectively coupled to ring gear 58 andsun gear 66 by clutch 74. Intermediate shaft 28 is selectively coupledto carrier 52 by clutch 76, selectively coupled to carrier 32 and ringgear 68 by clutch 78, and selectively coupled to ring gear 48 and sungear 56 by clutch 80. A suggested ratio of gear teeth for each planetarygear set is listed in Table 1.

TABLE 1 Ring 38/Sun 36 2.20 Ring 48/Sun 46 1.75 Ring 58/Sun 56 1.60 Ring68/Sun 66 3.70

As shown in Table 2, engaging the clutches and brakes in combinations offour establishes ten forward speed ratios and one reverse speed ratiobetween turbine shaft 14 and output shaft 18. An X indicates that theclutch is required to establish the speed ratio. An (X) indicates theclutch can be applied but is not required to establish the power flowpath. In 1^(st) gear, either clutch 78 or clutch 80 can be appliedinstead of applying clutch 76 without changing the speed ratio. When thegear sets have tooth numbers as indicated in Table 1, the speed ratioshave the values indicated in Table 2.

TABLE 2 A B C D E F 70 72 80 76 74 78 Ratio Step Rev X X X X −4.79 102%Park X X X 1^(st ) X X (X) X 4.70 2^(nd) X X X X 2.99 1.57 3^(rd) X X XX 2.18 1.37 4^(th) X X X X 1.80 1.21 5^(th) X X X X 1.54 1.17 6^(th) X XX X 1.29 1.19 7^(th) X X X X 1.00 1.29 8^(th) X X X X 0.85 1.17 9^(th) XX X X 0.69 1.24 10^(th  ) X X X X 0.64 1.08

Parking pawl 82 selectively couples output shaft 18 to the transmissioncase to prevent vehicle movement when the vehicle is parked. Unlikeshift elements 70-80, parking pawl 82 is designed to remain engagedwithout any external power once engaged. As illustrated in Table 2,shift elements 70, 72, and 80 may be engaged when the transmission is inPark. This combination does not establish a power flow path betweenturbine shaft 14 and output shaft 18. However, having several clutchesalready applied decreases the number of clutch engagements required totransition into reverse or 1st gear. Other combinations of three orfewer shift elements would also provide this benefit. Furthermore, it isadvantageous to have the elements of the transmission held againstrotation by hydraulic clutches as the parking pawl is released. This maybe accomplished by engaging five of the six shift element. Then, thepower flow path associated with either reverse of first gear isestablished by gradually releasing a shift elements. This sequenceavoids the sudden jerk that may accompany release of the parking pawlwhile a power flow path is engaged. For example, to transition from Parkto Reverse, elements D and F may be engaged prior to or simultaneouswith disengagement of the parking pawl placing the transmission in alocked state with elements A, B, C, D, and F all engaged. Then, elementC is gradually released to establish the Reverse power flow path.Similarly, to transition from Park to 1st, elements D and E may beengaged prior to or simultaneous with disengagement of the parking pawlplacing the transmission in a locked state with elements A, B, C, D, andE all engaged. Then, element C is gradually released to establish the1st gear power flow path.

FIG. 3 schematically illustrates a hydraulic control system suitable forthe transmission of FIG. 1 with the gearing arrangement of FIG. 2. Solidlines represent the flow of fluid and dashed lines represent informationsignals. A collection of fluid passageways connected to transport fluidsuch that the pressure is substantially equal at various locationswithin the collection may be called a hydraulic circuit. Slightvariation in pressure within a hydraulic circuit may occur due toparasitic viscous drag of flowing fluid. A hydraulic circuits may beconnected to another hydraulic circuit by an orifice that permits somefluid flow between the circuits but intentionally limits the flow rateand creates an intentional pressure differential when flow occurs.Hydraulic circuits may also be connected to one another by valves. Avalve may block flow between the circuits in some circumstances, permitfree flow with negligible pressure drop in other circumstances, andpermit limited flow with intentional pressure drop in yet othercircumstances.

Fluid supply subsystem 100 provides fluid at elevated pressure in threecircuits: a pump output circuit 102, a line pressure circuit 104, and anLP Ctrl circuit 106. The pressure in these circuits varies in responseto control signals from controller 26. Pump output circuit 102 and linepressure circuit 104 are designed to accommodate high fluid flow rateswith minimal parasitic pressure drop. Clutch control subsystem 108regulates the pressure in six clutch apply circuits, 110 through 120 toa pressure less than line pressure in response to signals fromcontroller 26. Each of the six clutch apply circuits routes fluid to theapply chamber of one of the six shift elements of FIG. 2 respectively.Park control subsystem 122 mechanically engages and disengages parkingpawl 82 in response to variations in the pressures in the clutch applycircuits. Converter/lube control subsystem 124 regulates the pressureand flow in a lubrication circuit 126, a torque converter clutch applycircuit 128, and a torque converter clutch release circuit 130. Thestructure and operation of each of these subsystems is discussed in moredetail below.

FIG. 4 schematically illustrates the fluid supply subsystem 100. Similarfluid supply subsystems are discussed in U.S. Patent ApplicationPublications 2013/0014498 and 2013/0017112 which are incorporated byreference in their entirety herein. Pump 20, which is driven by thetransmission input shaft, draws fluid from sump 22 and delivers thefluid to pump output circuit 102. Pump 20 is a positive displacementpump. Disregarding leakage, positive displacement pumps deliver acertain amount of fluid per revolution of the pump shaft regardless ofthe relative pressure at the pump inlet and pump outlet. The torquerequired to rotate the pump shaft increases as the pressure at the pumpoutlet increases relative to the pressure at the inlet. The amount offluid delivered per revolution is called the pump displacement. Thedisplacement of pump 20 varies within predefined limits based on thepressure in displacement decrease circuit 140.

During normal operation, anti-backflow valve 142 is open such that fluidflows freely from the pump outlet circuit 102 to the line pressurecircuit 104 and the pressure in the two circuits is substantially equal.The controller adjust the pressure in these two circuits by sending acommand to line pressure Variable Force Solenoid (VFS) 144. Fluid flowsfrom the pump out circuit 102, through an orifice 146, through a valveopening in line pressure VFS 144 and then into LP Ctrl circuit 106. Thepressure drop from the pump output circuit 102 to the LP Ctrl circuit106 varies depending upon the size of the opening in line pressure VFS144. The size of the opening in line pressure VFS 144 varies based onmovement of a spool. Electrical current from controller 26 creates amagnetic force on the spool tending to enlarge the opening. Fluid in theLP Ctrl circuit 106 acts on an area of the spool to create a forcetending to reduce the size of the opening. An equilibrium is reached atwhich the pressure in the LP Ctrl circuit 106 is proportional to theelectrical current.

Main regulator valve 148 adjusts the displacement of pump 20 in order tomaintain the pressure in pump out circuit 102 proportional to thepressure in the LP Ctrl circuit 106. Pressure in the LP Ctrl circuit 106generates a force on a spool in main regulator valve 148. Pressure inthe pump out circuit 102 generates a force on the spool valve in theopposite direction. When the pressure in the pump out circuit 102exceeds the pressure in the LP Ctrl circuit, the spool moves to allowflow from pump out circuit 102 to displacement decrease circuit 140.Pressure in circuit 140 causes a reduction in the flow rate from pump 20into the pump out circuit 102. Components fed by the pump out circuit102 and the line pressure circuit 104 establish a relationship betweenthe pressure in these circuits and the flow rate. Consequently, thereduction in flow rate results in a reduction in the pressure in pumpout circuit 102 until an equilibrium is reached.

When the vehicle is stopped, such as when waiting at a traffic light,powertrain controller 26 may shut off the engine to conserve fuel. Whenthe driver again demands torque by releasing the brake and depressingthe accelerator pedal, the controller restarts the engine. In order torespond quickly after the engine is restarted, it is important tomaintain some clutches in an engaged state. Fluid flow to maintain theseclutches is provided by electrically driven pump 150 which directlyfeeds line pressure circuit 104. During engine shutdown periods,controller 26 adjusts the pressure in line pressure circuit 104 bycontrolling the speed of the electric motor driving pump 150. Controller26 stops supplying current to line pressure VFS 144 causing the pressurein LP Ctrl circuit 106 to drop to ambient pressure. In response to thisreduction in LP Ctrl pressure, anti-backflow valve 142 closes to preventflow from line pressure circuit 104 to pump out circuit 102. Therefore,when the engine is shut down, the pressure in pump out circuit 102 dropsto ambient pressure.

FIGS. 5 and 6 schematically illustrate the clutch control subsystem 108.Controller 26 adjusts the torque capacity of each clutch by adjusting anelectrical current to a corresponding solenoid. During a shift, accuratecontrol of the torque capacity of the on-coming and off-going clutchesis very important. The relationship between changes in the electricalcurrent and changes in torque capacity is called the gain. If the gainis too high, then accuracy of torque capacity control suffers. Thetorque capacity of engaged clutches while in a fixed gear or holdingclutches during a shift must be maintained higher than the transmittedtorque in order to avoid clutch slip. Sometimes, these requirements arein tension with one another. For example, in reverse, the torquecapacity of brake A must be maintained at more than three and a halftimes the gearbox input torque. In 6^(th) gear, on the other hand, thetorque transmitted by brake A is less than 30% of the gearbox inputtorque. Brake A is the off-going element in a shift from 6^(th) gear to7^(th) gear. During this shift, which may occur at relatively lowgearbox input torque, a low gain is required. However, this same lowgain would not be suitable in reverse gear at relatively high gearboxinput torque.

FIG. 5 illustrates the components that control four of the six shiftelements of the gearing arrangement of FIG. 2, CL A 70, CL B 72, CL C80, and CL F 78. Each clutch apply circuit is controlled by thecombination of a Casting-Integrated Direct-Acting Solenoid (CIDAS) 160,162, 164, or 166 and a corresponding latch valve 168, 170, 172, or 174.Each CIDAS controls the pressure in a corresponding controlled pressurecircuit 176, 178, 180, or 182 in response to a control signal fromcontroller 26. Each latch valve connects a clutch apply circuit to acorresponding controlled pressure circuit when the pressure in thecontrolled pressure circuit is below a threshold and connects the clutchapply circuit to line pressure circuit 104 when the controlled pressureis above the threshold. This arrangement enables use of a low gainduring shift events and yet provides high torque capacity at othertimes. The thresholds and gains may vary among the various clutches.When a controlled pressure is commanded to zero, the CIDAS valveconnects the controlled pressure circuit to clutch exhaust circuit 184which provides a path for fluid to escape from the clutch apply chamberto de-stroke the clutch piston. Elevated exhaust circuit 186 provides asupply of fluid at very near ambient pressure. The structure andoperation of a CIDAS/latch valve combination is described in detail inU.S. Patent Application Publication 2013/0026401 which is incorporatedby reference in its entirety herein.

FIG. 6 illustrates the components that control the other two of the sixshift elements of the gearing arrangement of FIG. 2, CL D 76 and CL E74. Fluid flows from line pressure circuit 104, through an opening inClutch D CIDAS 190 into Clutch D apply circuit 116. The size of theopening varies depending upon the position of a spool in CIDAS 190. Anelectrical signal from controller 26 creates a magnetic force pushingthe spool in one direction tending to increase the size of the opening.Fluid in the clutch D apply circuit 116 acts on an area of the spooltending to push the spool in the opposite direction and reduce the sizeof the opening. Additionally, fluid in the clutch D feedback circuit 192acts on a second area also tending to reduce the size of the opening.The pressure drop between the line pressure circuit and the clutch Dapply circuit is related to the size of the opening. An equilibrium isreached at which the pressure in the clutch D apply circuit isproportional to the electrical current. The coefficient ofproportionality, or gain, is determined by gain control valve 194. Whenthe pressure in the LP Ctrl circuit 106 is above a threshold, gaincontrol valve 194 connects the clutch D feedback circuit 192 to theelevated exhaust circuit 186. In this condition, the gain is relativelyhigh because the pressure in the clutch D apply circuit acts only on thefirst area of the spool. When the pressure in the LP Ctrl circuit 106 isbelow the threshold, gain control valve 194 connects the clutch Dfeedback circuit 192 to the clutch D apply circuit 116. In thiscondition, the gain is relatively low because the pressure in the clutchD apply circuit acts only on both the first and second areas of thespool. Similarly, clutch E CIDAS 196 and gain control valve 194cooperatively control the pressure in clutch E apply circuit with twodifferent gains. The structure and operation of a combination of valves190, 194, and 196 is described in detail in U.S. Patent ApplicationPublication 2014/0182693 which is incorporated by reference in itsentirety herein. In an alternative embodiment, gain control valve 194could be controlled by a separate signal from controller 26. Blowoffvalve 198 exhausts the clutch exhaust circuit 184 from all six clutchesto the sump, maintaining a slight positive pressure such that thecircuit does not become evacuated.

FIG. 7 schematically illustrates the park control subsystem. A similarsystem is described in detail in U.S. Patent Application Publication2014/0284170 which is incorporated by reference in its entirety herein.The spool of park valve 200 is mechanically linked to the park mechanism82, such that movement in one direction engages the park mechanism andmovement in the opposite direction disengages the park mechanism. Aspring within the park mechanism biases the system toward engagement.Also, the pump out circuit 102 acts on an area of the spool which forcesthe spool toward engagement. Circuits 202 and 204 act on areas of thespool tending to push the spool in the disengagement direction. Theareas on which these circuits act are balanced such that pressure inboth circuit 202 and circuit 204 must be near the pressure in pump outcircuit 102 in order to push the spool into the disengaged position.When the pressure in clutch D apply circuit 116 is high, ball valve 206connects circuit 116 to circuit 202. Similarly, when the pressure inclutch B apply circuit 112 is high, ball valve 208 connects circuit 112to circuit 204. Thus, park may be disengaged by simultaneouslycommanding high pressure to clutch apply circuits 112 and 116.

Once the spool moves into the disengaged position, the valve connectsout of park circuit 210 to circuit 212. Ball valves 214, 216, and 218connect circuit 212 to one of clutch D apply 116, clutch F apply 120,clutch C apply 114, or clutch A apply 110, whichever has the highestpressure. Thus, circuit 212 is pressurized at close to line pressurewhenever at least one of these clutches is commanded to fully engaged.Whenever the pressure in out of park circuit 210 is higher clutch Dapply 116 or clutch B apply 112, ball valves 206 and 208 connect the outof park circuit to circuits 202 and 204 respectively. Thus, once park isdisengaged, it remains disengaged as long as at least one of clutches A,C, D, and F are fully engaged, even if the clutches that were engaged inorder to cause the transition are released. As shown in Table 2, everygear state involves engagement of at least two of these clutches.Furthermore, every shift in which one element is released and another isengaged would have at least one of these four clutches as a holdingclutch. As discussed above with regard to the fluid supply subsystem,the engine may sometimes be shut off while the vehicle is stationary. Anelectric pump maintains pressure in the line pressure circuit while theengine is shut down. Therefore, as long as full pressure is commandedfor at least one of clutches A, C, D, or F, the vehicle stays out ofpark during these engine shut down events. To re-engage park, all ofthese clutches must be commanded to a lower pressure, which can be donewithout completely disengaging the clutches.

FIG. 8 illustrates the portion of the lube and converter controlsubsystem 124 that controls the torque converter. The system is designedto operate a two-pass type torque converter. As the name implies, atwo-pass torque converter utilizes only two hydraulic circuits to i)feed fresh fluid to the converter, ii) return fluid from the converter,and iii) to control the torque capacity of the lock-up clutch. When thelock-up clutch is disengaged, fluid flows into the torque converter inTCC release circuit 130 and flows out of the converter in TCC applycircuit 128. On the other hand, when the lock-up clutch is engaged,fluid flows into the converter in TCC apply circuit 128 and flows out ofthe converter in TCC release circuit 130 with the pressure differencebetween these circuits controlling the torque capacity of the lock-upclutch. This contrasts with a three-pass type torque converter in whicha separate circuit is dedicated to each of the functions.

Controller 26 indicates the desired lock-up clutch torque capacity byadjusting an electrical signal. Fluid flows through an opening in TorqueConverter Mini-Direct-Acting solenoid valve 220 from pump out circuit102 to TCC Ctrl circuit 222. The valve controls the size of the opening,and therefore the pressure drop between these circuits, such that thepressure in TCC Ctrl circuit 222 is proportional to the electricalsignal. Under normal operating conditions, priority valve 224 connectsthe pump out circuit 102 to the converter feed circuit 226. Theconverter feed circuit 226 supplies fresh fluid to the torque converter.As discussed below, the converter feed circuit also supplies fluid to alubrication circuit. In circumstances where the pump is unable tomaintain the desired line pressure, priority valve 224 temporarilyreduces and may even shut off flow to converter feed circuit 226.Priority valve 224 determines that this condition exists based on thepressure in displacement decrease circuit 140. Recall that mainregulator valve 148 increases the pressure in this circuit when there isexcess flow available in order to reduce the flow rate. A pressure belowa threshold in the displacement decrease circuit 140 implies that themain regulator is requesting full displacement and the pump is still notgenerating enough flow. This can happen, for example, when a large flowrate is dedicated to moving a clutch piston to a stroked position.

Torque converter regulator valve 228 performs several functions, all inresponse to the TCC Ctrl pressure. Pressure below a threshold in the TCCCtrl circuit 222 implies that the lockup clutch should be disengaged. Inresponse, valve 228 i) connects converter feed circuit 226 to TCCrelease circuit 130, and ii) connects TCC apply circuit 128 to converterout circuit 230. As discussed below, converter out circuit 230 suppliesfluid for lubrication. When the pressure in the TCC Ctrl circuit 222 isabove the threshold, valve 228 i) connects the pump out circuit 102 tothe TCC apply circuit 128 through a variable size opening, ii) adjustthe size of the opening such that the pressure in the TCC apply circuitis proportional to the pressure in the TCC Ctrl circuit 222, iii)connects the TCC release circuit 130 to the sump 22, and iv) connectsthe converter feed circuit 226 to the converter out circuit 230.

FIGS. 9a and 9b illustrate a cross section of torque converter regulatorvalve 228. Spool 250 slides axially within a valve bore. Spool 250includes four spool lands 252, 254, 256, and 258 to define a number ofchambers. Chamber 260 is between a fixed wall on the left end of thebore and land 252. Chamber 262 is between land 252 and land 254. Chamber264 is between land 254 and land 256. Chamber 266 is between land 256and land 258. Finally, chamber 268 is between land 268 and a fixed wallon the right end of the bore. Compression spring 270 tends to push thespool toward the right. A number of openings in the side of the valvebore, called ports, connect particular chambers to particular hydrauliccircuits of the hydraulic control system. Which chamber(s) a particularcircuit is connected to may depend upon the position of spool 250. Also,the size of the port opening may depend upon the position of the spool250.

FIG. 9a illustrates the regulator valve with the spool in the positioncorresponding to an open torque converter. The TCC Ctrl circuit 222 isconnected to chamber 268 such that pressure in the circuit tends to pushspool 250 toward the left. When the TCC Ctrl pressure is below athreshold, spring 270 pushes spool 250 into the position shown in FIG.9a . In this position, both chambers 260 and 262 are vented to sump, sothe pressure in these chambers is negligible. Chamber 264 is connectedto both TCC release circuit 130 and converter feed circuit 226permitting fluid flow from converter feed circuit 226 to TCC releasecircuit 130 to release the bypass clutch and provide fresh fluid to thehydrodynamic torque converter. Chamber 266 is connected to bothconverter out circuit 230 and TCC apply circuit 128 permitting fluidexiting the torque converter in TCC apply circuit 128 to flow throughthe converter out circuit 230 into the cooler and lube circuit.

FIG. 9b illustrates the regulator valve with the spool in the positioncorresponding to a locked or slipping torque converter. Pressure in TCCCtrl circuit 222 is sufficient to overcome spring 270 to push spool 250to the left. Fluid flows into chamber 262 from pump out circuit 102 andfrom there to the TCC apply circuit 128. The size of the opening betweenpump out circuit 102 and chamber 262 depends upon the position of spool250. Due to pressure drop across this restricted opening, the pressurein chamber 262 is less than the pressure in pump out circuit 102. Land254 has a larger diameter than land 252 such that pressure in chamber262 tends to push spool 250 to the right. Spool 250 moves to anequilibrium position such that the rightward force from chamber 262 andthe spring force balance the leftward force from chamber 268. At theequilibrium, the pressure in chamber 262, and therefore the pressure inTCC apply circuit 128, is a function of the pressure in TCC Ctrl circuit222. TCC release circuit 130 is vented to sump through chamber 264.Fluid flows from converter feed circuit 226 to converter out circuit 230through chamber 266.

This one valve 250 accomplishes multiple functions associated withoperation of the two pass torque converter. In the released state ofFIG. 9a , i) fluid is routed from a flow source (converter feed circuit226) to the converter release circuit 130, and ii) fluid is routed fromconverter apply circuit 128 to a lubrication circuit via converter outcircuit 230. In the applied state of FIG. 9b , i) fluid is routed toconverter apply circuit 128 at a pressure based on the pressure in acontrol circuit (TCC Ctrl 222), ii) fluid is routed from the flow source(converter feed circuit 226) to the lubrication circuit via converterout circuit 230, and iii) fluid is exhausted from the converter releasecircuit 130 to sump. Accomplishing all of these function in a singlevalve avoids possible error states that could occur if the functions areperformed by multiple valves. When the functions are performed inmultiple valves, the switching from an applied state to a released stateor vice versa requires multiple valves to change position. If one of thevalves fails to change position, the resulting inconsistent connectionsamong circuits may starve the torque converter or the lubricationcircuit of fresh fluid. Valves may stick in position, for example, dueto contamination by small particles. Provisions to detect and mitigatesuch an error state add considerable complexity and cost to the controlsystem.

Valve 150 is designed to mitigate the error modes of a stuck valve. Ifvalve 150 sticks in either the applied or released position whencommanded to the opposite position, the circuits are connected in aconsistent state that will provide lubrication fluid. If the torqueconverter bypass clutch is released, such that heat is generated in thetorque converter, then the torque converter is supplied with fresh fluidto remove the heat. FIG. 9c shows the valve stuck in an intermediateposition. In this position, chamber 266 connects the converter feed tothe converter out circuit such that flow to the lubrication circuit isnot interrupted. Also, both chambers 262 and 264 are vented to sump,although via separate circuits. As a result, fluid from the torqueconverter will tend to drain to sump though one of these circuits and bereplaced with air through the other circuit. As the fluid drain out ofthe torque converter, the K-factor of the converter increases (theconverter becomes “looser”). The controller can detect this change bycomparing a measured turbine speed and impeller speed to a predictedspeed based on the torque level. In response to detecting this K factorchange after attempting to transition from applied to released, thecontroller may increase the pressure in the TCC Ctrl circuit to returnto the applied state. Similarly, if the error state is detected after anattempt to transition from released to applied, the controller mayreturn to the released state. The transmission may be operatedindefinitely in the released state, although fuel economy may beadversely impacted. If the vehicle come to a stop while the transmissionis being operated in the applied state, the controller may release on ofthe shifting clutches to enter a neutral state. A shifting clutch maythen be utilized as a launch clutch while the vehicle is driven to aservice facility.

FIG. 10 illustrates the portion of the lube and converter controlsubsystem 124 that controls flow from the converter out circuit 230 tothe lubrication circuit 126. Thermal bypass valve 232 determines whetherto connect the converter out circuit 230 directly to the lube circuit126 or whether to routes it through cooler 234 via cooler circuit 236.Thermal bypass valve is a passively controlled valve that routes thefluid based on the temperature in elevated exhaust circuit 186. When thetemperature is above a threshold, the fluid is routed through thecooler. When the temperature is in the normal operating range, the fluidis routed directly to lube circuit 126. In some applications, the fluidmay also be routed through the cooler when the temperature is below thenormal operating range. The cooler may be a heat exchanger betweentransmission fluid and engine coolant. Since the engine typically warmsup faster, the cooler may function as a heat source for the transmissionfluid during the warm-up period, providing faster warm-up. Since coldfluid has substantially higher viscosity, warming it up quicker reducesfuel consumption. The structure and operation of valve 232 is discussedin detail in U.S. patent application Ser. No. 14/282,051 filed May 20,2014 which is incorporated by reference in its entirety herein. Luberegulator valve 238 permits some fluid to flow from lube circuit 126 tosump 22 through a controlled opening in order to maintain a desirepressure in lube circuit 126.

While exemplary embodiments are described above, it is not intended thatthese embodiments describe all possible forms encompassed by the claims.The words used in the specification are words of description rather thanlimitation, and it is understood that various changes can be madewithout departing from the spirit and scope of the disclosure. Aspreviously described, the features of various embodiments can becombined to form further embodiments of the invention that may not beexplicitly described or illustrated. While various embodiments couldhave been described as providing advantages or being preferred overother embodiments or prior art implementations with respect to one ormore desired characteristics, those of ordinary skill in the artrecognize that one or more features or characteristics can becompromised to achieve desired overall system attributes, which dependon the specific application and implementation. As such, embodimentsdescribed as less desirable than other embodiments or prior artimplementations with respect to one or more characteristics are notoutside the scope of the disclosure and can be desirable for particularapplications.

What is claimed is:
 1. A transmission comprising: first, second, andthird shift elements engageable in combination to establish a reversetransmission ratio; a fourth shift element engageable in combinationwith the first and second shift elements to establish a forward launchtransmission ratio; first, second, third, and fourth hydraulic circuitsconfigured to engage the first, second, third, and fourth shift elementsrespectively in response to pressurization of the respective hydrauliccircuit; a parking pawl engageable to hold a transmission output shaftagainst rotation; and a park valve configured to disengage the parkingpawl in response to simultaneous pressurization of the first and secondhydraulic circuits and then to maintain the parking pawl in a disengagedstate in response to pressurization of either of the second or thirdhydraulic circuits.
 2. The transmission of claim 1 further comprising: afifth shift element; and a fifth hydraulic circuit configured to engagethe fifth shift element in response to pressurization of the fifthhydraulic circuit.
 3. The transmission of claim 2 wherein the park valveis further configured to maintain the parking pawl in the disengagedstate in response to pressurization of the fifth hydraulic circuit. 4.The transmission of claim 2 further comprising: a sixth shift elementengageable in combination with the first, second, and third shiftelement to establish the reverse transmission ratio and engageable incombination with the first, second, and fourth shift elements toestablish the forward launch transmission ratio; and a sixth hydrauliccircuit configured to engage the sixth shift element in response topressurization of the sixth hydraulic circuit.
 5. The transmission ofclaim 4 wherein the park valve is further configured to maintain theparking pawl in the disengaged state in response to pressurization ofthe sixth hydraulic circuit.
 6. The transmission of claim 5 furthercomprising: a controller programmed to command engagement of the first,fifth, and sixth shift elements in response to a shift selector being ina Park position and in response to movement of the shift selector fromthe Park position, command engagement of the second shift elementcausing disengagement of the parking pawl.
 7. The transmission of claim6 wherein the controller is further programmed to respond to movement ofthe shift selector into a Reverse position by commanding engagement ofthe third shift element and release of the fifth shift element.
 8. Thetransmission of claim 6 wherein the controller is further programmed torespond to movement of the shift selector into a Drive position bycommanding engagement of the fourth shift element and release of thefifth shift element.
 9. The transmission of claim 6 wherein thecontroller is further programmed to respond to movement of the shiftselector into the Park position by reducing torque capacities of thefirst, fourth, and fifth shift elements causing engagement of theparking pawl.
 10. The transmission of claim 1 further comprising:control valves configured to route fluid from a line pressure circuit tothe first through fourth hydraulic circuits and to control fluidpressures in the first through fourth circuits in response to commandsfrom a controller; a pump configured to supply pressurized fluid to theline pressure circuit in response to rotation of a transmission inputshaft; and an electrically driven pump configured to supply pressurizedfluid to the line pressure circuit when the transmission input shaft isnot rotating.
 11. A hydraulic control system comprising: first, second,third, fourth, and fifth clutch apply circuits; and a park valveconfigured to disengage a parking pawl in response to simultaneouspressurization of the first and second clutch apply circuits and then tomaintain the parking pawl in a disengaged state in response topressurization of any of the second through fifth clutch apply circuits.12. The hydraulic control system of claim 11 further comprising a sixthclutch apply circuit.
 13. The hydraulic control system of claim 11wherein: the park valve is configured to disengage park in response topressure in a first input circuit and a second input circuit and isconfigured to fluidly connect an out-of-park circuit to a third inputcircuit whenever park is disengaged; a first check valve configured toset the pressure in the first input circuit to a maximum of the pressurein the out-of-park circuit and a pressure in the first clutch applycircuit; a second check valve configured to set the pressure in thesecond input circuit to a maximum of the pressure in the out-of-parkcircuit and a pressure in the second clutch apply circuit; and aplurality of check valves configured to set the pressure in the thirdinput circuit to a maximum of pressures in the first, third, fourth, andfifth clutch apply circuits.
 14. A method of operating a transmissionhaving first through sixth shift elements comprising: commandingengagement of the first, second, and third shift elements in response toa shift selector being in a Park position; and in response to movementof the shift selector from the Park position, disengaging a parking pawlby commanding engagement of the fourth shift element.
 15. The method ofclaim 14 further comprising: in response to movement of the shiftselector into a Reverse position, commanding engagement of the fifthshift element and release of the second shift element.
 16. The method ofclaim 15 wherein engagement of the fourth shift element occurs afterengagement of the fifth shift element and before release of the secondshift element such that five shift elements are engaged as the parkingpawl is disengaged.
 17. The method of claim 14 further comprising: inresponse to movement of the shift selector into a Drive position,commanding engagement of the sixth shift element and release of thesecond shift element.
 18. The method of claim 17 wherein engagement ofthe fourth shift element occurs after engagement of the sixth shiftelement and before release of the second shift element such that fiveshift elements are engaged as the parking pawl is disengaged.
 19. Themethod of claim 14 further comprising: in response to movement of theshift selector into the Park position, engaging the parking pawl byreducing torque capacities of the first, second, and third shiftelements.